The proposed research will deal with the following questions: (a) Are cells of invasive tumors more mobile in aggregates than cells of non-invasive tumors? To answer this question, tumors will be produced in hamsters and invasive and non-invasive growths selected. Mobility in aggregates will be assayed as before, using the histological/autoradiographic methods previously published to determine intermixing. (b) The cellular events in the penetration of a malignant cell into a solid mass (aggregate) of cells will be examined. Specifically, penetration will be assessed quantitatively as described and the morphological features of this process detailed by scanning electron microscopy. (c) The role of extracellular matrix in control of cell movement will be studied. Specifically, collagen content and biosynthesis and collagenase activity will be examined in aggregates and correlated with cellular mobility.